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What is a gravity assist or a swingby for a spacecraft? I've heard about spacecraft gaining velocity from passing close to planets and being 'sling shot'-ed into a higher orbit. Where does this energy come from?

Gravity assist is used to speed up, slow down or change the direction of a spacecraft as it approaches a planet within our solar system. This saves fuel that would otherwise have to be used to make these changes.

Here are the basics! The spacecraft approaches the planet. Because the planet is moving too (while orbiting the Sun), it actually can boost the velocity of the spacecraft. What the spacecraft gains in velocity, the planet loses.

Here's a real example...Galileo increased its speed at Earth by 11,620 mph in 1990 and 8,280 mph in 1992. The Earth slowed down in its orbit by a speed of about 5 billionths of an inch per year from the two events.

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